Liquefied Natural gas, LNG, is primarily methane, propane and other heavier hydrocarbons. Producers of LNG until recently had considerable flexibility in the specifications of the gas that they liquefied, but there is now a trend toward tightening the composition requirements for LNG, specifically in North America which has seen the current standards for methane content approaching 100%. This means that in order to meet the new standards of these markets, producers of LNG are faced with not only the formidable problem of liquefying the fuel, but also using cryogenic fractionation to exclude the undesirable components such as helium, nitrogen, CO2, ethane, propane and heavier from the mixture.
The production of LNG is most applicable in situations where the source of the gas is an isolated field so far from markets for the gas that a pipeline cannot be economically justified. A gas liquefaction plant could then be located at or near the stranded gas field where the well head gas could be purified by removal of contaminants such as sulfur and CO2, then chilled and fractionated to remove light overhead gas components plus the heavier hydrocarbons, leaving a cryogenic liquid product that is almost pure methane. The LNG can be transported, usually by ship, to waiting markets where it can be vaporized, compressed and distributed to waiting markets by pipeline. The conditioning of the gas at source meets the stringent requirements of pipeline companies and by consumers of natural gas. Most liquefaction facilities have been located around the Atlantic and Pacific basins to serve markets in Europe, North America and Japan, but recently there have been new LNG facilities established in the Middle East to serve markets in Europe. Another use for LNG technology is for peak shaving to meet periods of high demand for natural gas. Many small countries far from markets for their gas have benefited economically from the strategy of exporting their surplus gas in the form of LNG.
In the conventional LNG process, raw gas entering the liquefaction plant must first be treated for removal of sulfur compounds, CO2 and Water. Specifications for natural gas specify that sulfur compounds, if any, must be totally removed except for a few PPM. Carbon dioxide must be removed so that it does not freeze and form a solid (dry ice) in the cryogenic equipment downstream. Water vapor must be removed to less than one part per million to avoid formation of gas hydrates. The conventional LNG process uses amine to remove sulfur compounds and CO2 followed by a fixed bed desiccant process to remove water.
The most practical way to transport natural gas is by pipeline, but, if a pipeline cannot be economically justified, then alternate methods must be used. The problem in transferring gas from one location to another in any type of container is the volume of the gas. Even a very small quantity of gas occupies a very large volume. This is the reason why the LNG process was developed. By liquefying the gas at −255 F (−160 C) and one atmosphere its volume can be reduced by a factor of 600. The LNG thus produced is a clear colorless liquid having a specific gravity of 0.45.
Liquefaction makes it practical to ship the gas as LNG by tanker. LNG tankers are huge double hulled ships specifically designed to contain the LNG within the inner hull of the vessel. Then cargo must be maintained at −255 F (160 C) at one atmosphere by an on board refrigeration system. The LNG tanks on board the ship are usually huge spheres, although other types of containment can be used. LNG ships are huge, typically containing up to 2,825 MMSCFD (80 000 000 SM3) of natural gas transported in liquefied form as 5,000,000 cubic feet (140,000 M3) of LNG on board the ship. Because of the huge size of the LNG containment vessels it is not practical to design them as pressure vessels. They are designed to transport the liquefied gas at atmospheric pressure which means the cargo must remain chilled to −255 F. If the gas pressure could be several atmospheres the shipping temperature could be somewhat higher.
Chilling the gas to its liquefaction temperature involves mechanical refrigeration and isenthalpic and/or isentropic expansions by means of let down valves or turbo expanders. Unwanted light gases are eliminated by cryogenic fractionation as are ethane and heavier fractions. Because temperatures are so low in the conventional process special refrigerant systems are required such as the nitrogen cycle or the ethylene vapor/liquid cycle. Standard industrial refrigeration equipment normally cannot be used. Fractionation to produce an LPG product that is essentially pure methane is a major challenge and the process is complex to make it efficient. The complexity is justified by the need for energy efficiency in a large scale plant that produces LNG by the shipload. In those very large LNG plants, energy efficiency is a vital concern.
Using the conventional LNG Process is not practical for small scale. LNG plants because the cost and complexity of the process makes the cost of the LNG product too expensive.